The present invention relates to surgical knives for making a controlled depth of cut and more particularly to knives for ophthalmic surgery where the control of the depth of cut can be critical to achieving the desired surgical result.
Many surgical procedures require that the depth of cut be controlled and surgery on the eye is one area where these procedures and the depth of cut are most critical. A controlled partial thickness (lamellar) eye wall incision is often needed for cataract surgery, peripheral iridectomy, glaucoma filtering surgery, lamellar keratectomy, radial lamellar corneal incisions for correction of myopia, retinal detachment surgery and other procedures. Without a knife that is capable of making a controlled depth incision the surgeon must free-hand the depth of the lamellar incision. This is particularly dangerous in that if the incision becomes too deep, vitreous or aqueous fluid may leak from the eye and intraocular structures may be damaged, resulting in possible blindness.
By using the free-hand depth of control method, it is often necessary for the surgeon to go back over an incision gingerly and repeatedly to achieve the desired depth of cut. Such multiple passes with the knife are time consuming, create a more ragged wound and distract the surgeon from the proper incision location. Additionally, as the surgeon draws the knife blade through the tissue and toward himself by bending the fingers, the angle of the leading edge of the knife handle to the tissue being cut becomes less acute. Alternately, when the incision is made by pushing the blade away from the surgeon by extending the fingers, the angle of the leading edge of the knife handle to the tissue being cut increases beyond 90.degree.. In either of these instances the depth of cut can vary drastically. Ideally, the knife should maintain a relatively uniform depth of cut throughout the entire incision despite the changes in the angle of the handle and the blade itself to the tissue being cut.
One type of surgical knife in common usage for making eye wall incisions consists of a simple cylindrical plastic handle which tapers down to a rounded tip having a diameter of approximately 2 millimeters. A sharpened steel blade is embedded in the rounded tip and extends approximately 2 millimeters from the rounded plastic tip of the knife handle. This type of knife is designed for surgical procedures requiring a controlled depth of cut. However, for making eye wall incisions the blade extends much further beyond the tip of the handle than the 0.5 millimeters average cut depth for lamellar eye wall incisions. Thus, the surgeon must rely on the free-handing techniques that he has developed in previous operations. In other surgical procedures where a 2 millimeter depth of cut is acceptable, the free-handed method must still be relied upon since the rounded tip of the handle may not be an acceptable blade guard, i.e., it may follow the blade into the incision.
A second type of surgical knife in common usage is the Beaver knife disclosed in U.S. Pat. No. 4,074,431 entitled "Surgical Knife Assembly, Surgical Blade and Method of Manufacture Thereof." The blades in this knife are thin (approximately 0.1 millimeters) allowing for fine incisions with a minimal amount of pressure and pointed to allow easy entry into the tissue. However, these knives have the same shortcomings as the one discussed above in that the blade is not guarded to afford a controlled lamellar depth of cut.
A guarded surgical knife is disclosed in U.S. Pat. No. 3,945,117 entitled "Surgical Blade With Adjustable Blade Guard." The design disclosed in this patent presents several problems in application. As the knife is angled away from the vertical position either by the surgeon drawing it toward him or pushing it away, the bearing surfaces of the guard rest against the surface being cut with the depth of cut varying by approximately 20%. In addition, the plastic blade guard projects beyond the leading and trailing edges of the blade thus reducing visibility of the blade and the surface being cut. In this knife, the blade is relatively thick (approximately 0.6 millimeters) in its flat portion. If this blade were used for making a lamellar incision of 0.5 millimeters in depth, the width of the incision would approach its depth, creating an undesirable furrow effect. Furthermore, the blade is not pointed, preventing easy entry at a precise location. Finally, since the knife is not preassembled and ready to use, the four components (blade, guard, handle, and guard adjuster) must be manipulated to adjust the extension of the blade from the end of the blade guard to provide the desired depth of cut. This manipulation is very time consuming considering that for eye surgery one normally is dealing with tolerances of 0.1 millimeters.
It would be desirable to have a surgical knife for making eye wall incisions wherein the blade is less than 0.2 millimeters thick at its thickest point, the blade being preset into a handle designed to provide a maximum of 10% depth of cut variation as the knife is pushed away from or pulled toward the surgeon, there is a built-in blade guard to ensure that the depth of cut does not vary more than the desired 10%, and that visualization of the blade is maximized during the surgery. The surgical knife of the present invention achieves each of the above desired results.